An automatic transmission has a speed change apparatus for shifting between a plurality of shift speeds by engaging and disengaging a plurality of friction engagement elements constituted by clutches and brakes. A pump is provided that is driven to rotate via a torque converter. By using oil discharged by the oil pump, hydraulic servos of the clutches and brakes are controlled, and the friction engagement elements of the clutches and brakes are lubricated together with gear elements of a speed change gear unit and so on. Conventionally, an inner rotor of the oil pump engages with a notch formed at one end of an impeller hub that is integrally connected to a pump impeller of the torque converter, and thus the inner rotor is driven to rotate integrally with the pump impeller.
In a type of automatic transmission, as described in Japanese Patent Application Publication No. JP-A-11-132160, for example, an engaging groove (notch) 12a that engages with a driving projecting portion (projection) 17a projecting from the inner periphery of an inner rotor 17 of an oil pump 11 is formed at one end of an impeller sleeve (impeller hub) 12 connected to a pump impeller 13. Engagement between the driving projecting portion (projection) 17a and the engaging groove (notch) 12a is performed by passing the impeller sleeve (impeller hub) 12 through an oil seal attached to an oil pump housing (oil pump body) 15a, and a dust lip for preventing foreign matter from infiltrating a transmission case is provided at one end of the oil seal.